For beginner piano: Vexilla Regis prodeunt

Which is also used on September 14th for the Exaltation of the Holy Cross, as well as (formerly) on May 3rd for the finding of the same. This hymns also shows up in the movie The War of the Vendée; incidentally, if I remember correctly, in both its Monastic and Roman versions.

Chant notation for this hymn can be found in the Liber Usualis, p. 575.

Oh, and one last use: in older chant books, such as the 1954 Liber Brevior, one may see that this chant was also formerly sung on Good Friday for the procession bearing the Blessed Sacrament to the altar.